This invention relates to an image forming device, such as a copier or a laser beam printer (LBP), using a constant-velocity joint.
A known image forming device includes a photoconductor drum rotated in one direction, and an electrifier, a light exposure means, a developing unit, and an image transfer roller which are provided around the photoconductor drum.
In such an image forming device, an electrostatic latent image is first formed on the photoconductor drum by exposing the drum to image light based on image information with the light exposure means, after uniformly electrifying the entire outer periphery of the photoconductor drum by means of the electrifier. Then, a toner is supplied onto the electrostatic latent image from the developing unit to form a toner image on the photoconductor drum. The toner image thus formed is then transferred onto recording paper by the image transfer roller, with the recording paper being fed at the same speed as the peripheral speed of the photoconductor drum.
In such an image forming device, if the mounting position of an image forming unit including the photoconductor drum is not accurate enough, the rotational speed of the photoconductor drum could fluctuate while the drum rotates once. For example, due to dimensional errors of the image forming unit or parts of the main body of the device supporting the image forming unit (such as frames and slide rails), the drum shaft of the photoconductor drum may be vertically or horizontally displaced from the dive shaft of the motor. Also, even if the drum shaft and the drive shaft of the motor are initially aligned with each other, after the image forming device is repeatedly mounted to and dismounted from the main body of the device, mounting errors may accumulate gradually.
In either case, the shaft of the photoconductor drum (which is hereinafter referred to as the rotary member shaft) will become inclined relative to the drive shaft of the driving source, thus causing fluctuations in rotational speed of the photoconductor drum.
If the rotational speed of the photoconductor drum fluctuates per rotation of the drum, an electrostatic latent image formed on the photoconductor drum by exposing the drum to light with the light exposure means will expand or shrink, which in turn causes expansion or shrinkage of a picture image formed on the transfer material by transferring a toner image, thus blurring the picture image. Thus, since fluctuations in the rotational speed of the photoconductor drum makes it difficult to obtain high-quality picture images, it is necessary to rotate the photoconductor drum at a constant speed.
For this purpose, Japanese Patent Publication 2007-256492A proposes to couple the rotary member shaft of the photoconductor drum to the drive shaft of the driving source through a tripod type constant-velocity joint, thereby preventing expansion and shrinkage of a picture image formed on a transfer material due to fluctuations in rotational speed of the photoconductor drum.
The constant-velocity joint disclosed in this publication comprises an outer ring and a tripod member mounted inside of the outer ring. The outer ring has three axially extending track grooves formed in the inner periphery thereof at angular intervals of 120 degrees. The tripod member has three protrusions slidably inserted in the respective track grooves such that torque is transmitted between the outer ring and the tripod member through the protrusions. The outer ring has bulges between the respective adjacent pairs of track grooves. Each bulge is formed at the distal end portions thereof with a pair of tapered surfaces. Each of the protrusions of the tripod member has a pair of tapered surfaces on the front side thereof.
This constant-velocity joint allows the axis of the rotary member shaft and the axis of the drive shaft to be inclined relative to each other with a relatively large angle therebetween, provided the two axes intersect each other in or near the joint. However, this constant-velocity joint does not allow the above two axes to be inclined relative to each other with a relatively large angle, if the above two axes intersect each other at a position remote from the joint. Also, this constant-velocity joint does not allow too large an offset between the above two axes if the axes are parallel to each other.
For rotary members of an image forming device other than the photoconductor drum which are rotated about their axes under a driving force, such as electrifying rollers, developing rollers, stirring rollers, it is also necessary to minimize fluctuations in rotational speeds of these rotary members per rotation in order to form high-quality picture images.
It is also desired that the rotary member shafts of these rotary members be smoothly connected to the corresponding drive shafts.